Need help tracking down intermittent misfire

[DrCharles]

My '93 525i auto (M50TU) suddenly developed a strange misfire one day. It always starts promptly and idles smoothly, cold or hot, even though it has 336K miles.

But at every cold start, and if the engine is shut off even for a few minutes when warm, and only from off-idle to about 1500 rpm, the engine is really rough. It feels like at least one cylinder is misfiring. Above 1500 the car drives normally.

Drive for 1-2 minutes and it's fine for a while, but it also sometimes comes back while driving at low rpm.
The tach and temperature gauge are functioning normally at all times. No CEL.

Today I changed the valve cover gasket because there was some oil in the spark plug tubes. I cleaned the oil out, and from the plug boots and insulators too, and put it all back together. All plug tips are a light tan with equal gaps. No ash or oil fouling. Only using 1 qt every 3K miles or so. No change to the misfire though.

Where should I be looking first? Intake and coolant temp sensors and their wiring? :confused I would expect a vacuum leak to interfere with the idle but it's always smooth.

[DrCharles]

I plugged in my Peake R5 tool and was surprised to find no stored fault codes. I thought there'd at least be one or two!

[fo3]

Light tan on the plugs says lean to me. These run very rich when cold and are prone to flooding if "moving out of the garage to get the lawnmower" etc.
Brown would be normal if lots of highway, but a light tan is not normal, not for anything and especially not a m50b25.

Smoke test it.
E: cold idle is smooth even when there's a vac leak because these run so rich in the first place, so a vac leak doesn't hurt cold idle or even warm idle, it hurts "just off idle".

E: I'm not here a lot so to fill in other thoughts:
Get an EVAP smoke tester and pay attention to under the manifold and fuel p reg line.
If nothing found move onto spark plug boots - if loose fitting and full of oil replace them
check fuel pressure too, consider injectors being clogged.

I had to sell my E34, I was still holding onto the dream of dropping into the USA and finding another and doing a coast to coast cruise in one, but I'm stuck down here in my 4x4 safari/patrol for now. Going to have a look around coast to coast through all the desert gorges and waterfalls down here in that for a little while.

[E34 Lives]

Rough idle only at low rpm might indicate vacuum leak, check braking system vacuum hoses or possible brake booster diaphragm failure, as you always step on brake to restart car and that is where the rough idle occurs and goes away after driving at higher rpm.

If it is brake booster diaphragm failure, smoke test will not identify the leak. Instead pumping brakes aggressively while engine is running in drive to see if it gets rough enough to kill off the engine, if it does then you found your brake booster vacuum leak. Have the brake booster rebuilt as new one is most likely NA and used part is just as prone to failure.

[DrCharles]

fo3, maybe Australian cars of this age do not have a catalytic converter? In the US they do, and the mixture has to be kept quite close to stoich (14.7) for the minimum emissions. The M50TU definitely does not run "super rich" (well, maybe at open-loop WOT to maximize power).
A very light tan color on the plug noses after 25k miles is also perfectly normal. Some traces of engine oil, ash, gas additives inevitably deposit on the nose and leave a slight coloration. I'm not talking about brown or black...

Anyway. With no codes I concluded that the vibration had to be from something rotating and imbalanced, not a misfire! :confused
So I grasped the fan and wiggled it back and forth. No looseness there. But I spun the fan while checking the viscous clutch tension - and what do you know, one blade was missing entirely, broken off right at the root! :eek:

I removed the clutch and fan, started it up - eureka. It's cured. :cool
New fan on the way from FCP Euro. No damage to the core, hoses or belts that I could find. Also no idea where the blade exited the engine compartment. Oh well.

[E34 Lives]

Great to hear it was a simple fix, good time to also replace fan clutch while you are in there.

[shogun]

So I grasped the fan and wiggled it back and forth... I spun the fan while checking the viscous clutch tension - and what do you know, one blade was missing entirely, broken off right at the root! It's cured. ..No damage to the core, hoses or belts that I could find. Also no idea where the blade exited the engine compartment. Oh well.
that was lucky, often a broken off blade can cause a lot of damage